The present invention relates to the field of fluids for fracturing subterranean formations, such as the formations surrounding oil or gas wells, to increase their permeability, and thereby increase the potential flow of recoverable hydrocarbons therefrom.
In order to improve the recovery of hydrocarbons from oil and gas wells, the subterranean formations surrounding such wells can be fractured. That is, fluid can be introduced into the formations through the well bores, under high pressure, to open the pores and cracks in the formations. The fluids preferably are loaded with proppants, which are particles of hard material such as sand, to wedge open cracks or pores in the formation, and thereby permanently increase permeability and hydrocarbon flow. Therefore, the fracturing fluids are preferably of high viscosity, and therefore capable of carrying effective volumes of proppant material.
Preferably, the fracturing fluid is susceptible of viscosity drop, so as to be easily removed from the well after fracturing. The fluid should also be fairly non-toxic to the well. That is, it should not leave residue that will form a cake to clog the well, or that will leave undesirable traces in the recoverable hydrocarbons that may be expensive to remove.
Fracturing fluids in common use include aqueous gels and hydrocarbon gels, found by the introduction of cross-linkable polymers into an aqueous or hydrocarbon fluid, followed by the catalyzed cross-linking of the polymers, and the subsequent breaking of the gels, for instance by pH adjustment. These types of systems are quite effective, but require extensive well clean up.
It has been proposed, for instance in U.S. Pat. No. 5,551,516, to provide a fracturing fluid with good viscosity and little residue by combining an inorganic salt stabilizer, a surfactant/thickener and an organic salt or alcohol. The fluid may also contain a gas, and thereby be in the form of a foam. The fluid disclosed in U.S. Pat. No. 5,551,516 develops viscosity of between 150 and 200 cp @ 170 secxe2x88x921 at temperatures of about 40xc2x0-50xc2x0 C., and surfactant loadings of up to about 5%.
Similar results for viscoelastic surfactant fluids utilizing a single surfactant have been reported by Brown et al in xe2x80x9cUse of Viscoelastic Carrier Fluid in Frac-Pack Applications, SPE 31114 (1996) and Stewart et al,xe2x80x9cUse of Solids-Free Viscous Carrying Fluid in Fracturing Applications: An Economic and Productivity Comparison in Shallow Completionsxe2x80x9d SPE 30114 (1994).
The object of the present invention is to provide a fracturing fluid utilizing an anionic surfactant in combination with a cationic surfactant, which at total loadings of up to about 6% surfactant delivers viscosity at 10xc2x0-70xc2x0 C. in the range of 20-500 cp @ 100Sxe2x88x921.
The applicant has found that a combination of an anionic surfactant, such as sodium xylene sulfonate, with a cationic surfactant such as N, N, N, trimethyl-1-octadecamnonium chloride will form a viscoelastic gel very quickly, and can be mixed batch-wise in advance, or on-the-fly for use as a fracturing fluid. Inorganic salts may be added as stabilizers, but these are not strictly necessary.
In a broad aspect, then, the present invention relates to a fracturing fluid for use in fracturing subterranean formations surrounding oil and gas wells comprising water, an anionic surfactant, and a cationic surfactant.